You need to disable specularity, build the cubemaps, switch to whichever mode you're not already in, reload the map, build the cubemaps again, re-enable specularity, and then reload the map again to test it. Whatever combination of commands you feel most comfortable with that does all of that should do the trick. I've gotten used to exiting to the main menu to switch modes or toggle specularity and then get back in the map from the Create Server menu, but that's more of a personal ritual.

The easiest way to tell if you've successfully built cubemaps in both modes is that in LDR mode, the process will go very quickly and involve screenshots flashing so quickly that you can't tell what any of them are, and in HDR mode each shot will fade from an overexposed image to a normal looking one and the game will probably appear to freeze eventually. If you've seen both, you've built both and you only need to test in one mode to make sure they were committed to the map.

I have only ever seen this issue by incorrectly built cubemaps. Double and triple check that you are following Crash's commands.

IF this doesn't work, try compiling your map with JUST LDR. If the white cubemaps are fixed, it IS 100% DEFINATELY an issue with your command order.

Personally, I use these commands in this order:

mat_specular 0 - this turns off reflections so that the purple reflections are not baked into your cubemaps
mat_hdr_level 2 - this makes sure you're in HDR mode
map <mapname>
buildcubemaps
disconnect
mat_hdr_level 0 - change to LDR
map <mapname>
buildcubemaps
disconnect
mat_hdr_level 2 - Back to HDR so you can enjoy normal lighting for screenshots and such
mat_specular 1 - Turn reflections back on
mat_reloadallmaterials - Reload all the materials in the game, this WILL take a while. It is imporant so that you can see your new cubemaps.